Microalgae represent the main source of biomass production in an aquatic ecosystem which also possess system for generation and elimination of reactive oxygen species (ROS). Marine water receives increased number of anthropogenic chemicals from point and non-point sources. ROS concentration is dynamic in nature, that enhanced by the chemical sources in the marine environment and later it induces the oxidative stress in the marine organisms such as microalgae. In order to understand ROS changes with respect to seasonal fluctuation of marine pollutants and their impact on microalgal communities, two regions of Southeast coast of India were selected i.e., Tuticorin – a pollution impact site and Rameshwaram – the reference site. In this study microalgae distribution during three seasons i.e., summer, monsoon and winter were analysed. Quantification of physiochemical and heavy metal ions were analysed. ROS such as superoxide radical, hydrogen peroxide and hydroxyl radicals were measured using Electron Spin Resonance. Species richness, dominance and cell density of microalgae were calculated. All season data analysis showed significant reduction of species distribution and diversity in both sites. The species richness i.e Shannon- wiener diversity was high in the reference site, where as the dominance simpson diversity was high in impact size showed that the pollutants decreased the richness increase the dominance of some microalgal species. Correlation between the chemical parameter and microalgal distribution showed that the physiochemical parameters like Alkalinity, Phosphorus, Nitrogen compounds, BOD, DO were determined the microalgal density, species richness and dominance in both sites. Dynamic changes of heavy metals ions such as Hg, Cd, Cr, Fe, Zn and Al with the other marine in an chemicals impact site during different season’s sites enhance the ROS production. It was found that the micro algal communities were highly affected by chemicals in polluted marine water and that could enhance the oxidative stress.